From the viewpoint of elevator safety, the machinery brakes of an elevator are a critical component. The functioning of the machinery brakes of an elevator must also be monitored and the elevator must be removed from use when a machinery brake is out of order.
In many modern elevator systems a frequency converter is used to control the elevator motor and the elevator motor is driven with a pulse-width-modulated signal. Particularly in these types of elevator systems, adjustment of the deceleration and speed of an elevator car is generally implemented by adjusting the frequency of the frequency converter or the width of the active-pulse window. A machinery brake only needs to be used in an emergency situation. In addition, a machinery brake is generally engaged when an elevator car has stopped at the point of a floor level for taking passengers or a load on board or for letting it/them out of the elevator car. In this type of elevator system a sudden failure of a machinery brake is at least theoretically possible.
Testing of a brake for possible failure is a part of the monitoring of the operation of brakes. Failure of a machinery brake is involved e.g. if/when the machinery brake does not stop the elevator car or is not able to hold the elevator car in position, but also if/when the machinery brake does not open when driving with the elevator machine or when torque is produced with the elevator machine. It is obvious that if/when the machinery brake does not stop the elevator car a dangerous situation is at hand. But a dangerous situation might also arise if the machinery brake does not open sufficiently, because the machinery brake might become broken e.g. as a result of overheating, in which case adequate braking force will not be obtained from it in an emergency situation. If when driving the motor against the brake the braking force suddenly decreases, the elevator car can rush out of control.
Numerous solutions are known in the art for measuring the stopping force of a machinery brake.                The international patent application publication WO 2005/066057 A2 of the applicant presents a method and a system for testing the condition of the brakes of an elevator. In the starting situation the elevator car is provided with an additional weight and a drive device is used for moving the elevator car upwards. The torque needed for making it move upwards is measured. After this the additional weight is removed. In the monitoring to be implemented at regular intervals, it is endeavored to drive the elevator with the brake on in a starting situation with a measured torque. If the elevator moves, it can be deduced that the brake has failed.        Otis Elevator Company's international patent application publication WO 2007/094777 A2 presents a method wherein the elevator car is stopped near a position switch and the brake of the elevator is closed. After this drive motor of the elevator is driven for pulling the elevator by means of a rope. If the elevator car moves in such a way that the state of the position switch changes, it is deduced that the brake is broken.        The invention in the method described in AG's European patent application publication EP 2 460 753 A1 closes the brake of an elevator and increases the torque of the motor until the elevator car moves. The torque is compared to a threshold value and if the threshold value is smaller than the limit value it is deduced that the brake has failed.        
Many fewer methods are known in the art, however, for testing sufficient opening of machinery brakes. In some elevator systems brake switches have been used, which brake switches change their state when the brake opens, i.e. when the brake shoe detaches from a rotating part of the machine. The power supply to the electric motor of the hoisting machine is disconnected if there is no change in state of the brake switch. Brake switches are expensive and subject to failure. For example, the switch contacts of a brake switch can oxidize.